We are studying four membrane problems: 1. In order to see whether the lactone ring is implicated in the ouabain inhibition of red cell K transport, we studied the effect of angelica lactone on K uptake. Preliminary experiments indicated that angelica lactone acts like ouabain in several respects. 2. Membrane binding of phloretin was studied becuase this molecule is a specific inhibitor of both the glucose and anion transport systems as well as having a generalized membrane interaction which modifies passive diffusion of small nonelectrolytes. Two binding sites for phloretin were found: a high affinity site which appears to be the protein site involved in the specific inhibitions, and a low affinity site which appears to be the site responsible for the generalized effects on passive diffusion, and may be attributed to the membrane lipids. 3. The effect of dog red cell shrinking on the interaction of 2,3-diphosphoglycerate (2,3-DPG) with the membrane has been studied by p31 nuclear magnetic resonance. There appears to be an important component of 2,3-DPG binding which is modulated by the shape and area of the intact cell membrane. 4. Nonelectrolyte transport across spherical lipid bilayers of egg phosphatidyl choline has been studied. The three factors governing permeation in this system are: partition coefficient, diffusion across the interface and diffusion within the bilayer. The specific contribution of such steps has been separated and the apparent free energy of activation of the two diffusion processes has been computed.